Controlling Cation Distribution and Morphology in Colloidal Zinc Ferrite Nanocrystals

This paper describes the first synthetic method to achieve independent control over both the cation distribution (quantified by the inversion parameter x) and size of colloidal ZnFe2O4 nanocrystals. Use of a heterobimetallic triangular complex of formula ZnFe2(μ3-O)(μ2-O2CCF3)6(H2O)3 as a single-source precursor, solvothermal reaction conditions, absence of hydroxyl groups from the reaction solvent, and the presence of oleylamine are required to achieve well-defined, crystalline, and monodisperse ZnFe2O4 nanoparticles. The size of the ZnFe2O4 nanocrystals increases as the ratio of oleic acid and oleylamine ligands to precursor increases. The inversion parameter increases with increasing solubility of the precursor in the reaction solvent, with the presence of oleic acid in the reaction mixture, and with decreasing reaction temperature. These results are consistent with a mechanism in which ligand exchange between oleic acid and carboxylate ligands bound to the precursor complex influences the degree to which the reaction produces a kinetically trapped or thermodynamically stable cation distribution. Importantly, these results indicate that preservation of the triangular Zn–O–Fe2 core structure of the precursor in the reactive monomer species is crucial to the production of a phase-pure ZnFe2O4 product and to the ability to tune the cation distribution. Overall, these results demonstrate the advantages of using a single-source precursor and solvothermal reaction conditions to achieve synthetic control over the structure of ternary spinel ferrite nanocrystals.

. Crystal data and structure refinement for 1.   Figure S18. 19 F NMR spectra characterizing cluster 1 and its ligand exchange with oleic acid .. 22 Figure S19. Plots of the Fe 2p3/2 region of the x-ray photoelectron spectra of ZnFe2O4 nanocrystals synthesized in two different reaction mixtures and at two different temperatures .. 23

Hot Injection Procedure
In a typical reaction, oleylamine (OAm, 2.7 mmol), oleic acid (OA, 2.7 mmol), and solvent (9 mL) were degassed at 100 °C under vacuum for 30 min and then heated to 230 °C. The precursor 1 (0.025 mmol) was dispersed in benzyl ether (1 mL) and injected into the solution of the hot mixture. The reaction was maintained at 230 °C for 1 h. The dark mixture was allowed to cool, then it was purified with three cycles of precipitation with acetone followed by centrifugation. This procedure was also performed at a reaction temperature of 290 °C.

Heat-up Procedure
In a 25 mL three-neck round-bottom flask equipped with a reflux condenser, precursor 1 (0.025 mmol) was mixed with oleylamine (OAm, 2.7 mmol), oleic acid (OA, 2.7 mmol), and benzyl ether (10 mL) forming a dark red solution. This mixture was degassed by alternating between vacuum and N2 flow for 3 times at room temperature, followed by heating under vacuum to 110°C for 1.5 h. Afterward, the mixture was heated (25°C/min) to 230°C under N2. The solution did not change color. This procedure was also performed at a reaction temperature of 290 °C.

H and 19 F Nuclear Magnetic Resonance (NMR)
All 19 F NMR experiments were performed at room temperature on a Bruker 400 MHz spectrometer. All 1 H NMR spectra were collected on a Varian 500 MHz spectrometer. Chemical shifts are reported in parts per million (ppm).

Conversion of the XRD pattern taken using Molybdenum Ka to Copper Kα
To convert a pattern obtained from diffraction of X-rays generated from Mo Ka emission to the equivalent pattern for diffraction of X-rays generated from Cu Ka emission, the data first needs to be converted into Q-space using equation S1, where lMo and qMo are the wavelength Bragg angles for diffraction of Mo Ka emission, respectively.
The Bragg angles for diffraction of the Cu Ka radiation, qCu, can then be calculated by rearranging equation S1 and replacing lMo = 0.71073 Å with lCu = 1.54148 Å to generate equation S2.
We note that this conversion is only appropriate for comparing positions of diffraction peaks. Any analysis of line shapes should be carried out using the originally collected data.

Data collection
A crystal (0.217 x 0.062 x 0.027 mm 3 ) was placed onto a thin glass optical fiber or a nylon loop and mounted on a Rigaku XtaLAB Synergy-S Dualflex diffractometer equipped with a HyPix-6000HE HPC area detector for data collection at 100.00(10) K. A preliminary set of cell constants and an orientation matrix were calculated from a small sampling of reflections. 1 A short pre-experiment was run, from which an optimal data collection strategy was determined. The full data collection was carried out using a PhotonJet (Cu) X-ray source with frame times of 0.51 and 2.04 seconds and a detector distance of 31.2 mm. Series of frames were collected in 0.50º steps in w at different 2q, k, and f settings. After the intensity data were corrected for absorption, the final cell constants were calculated from the xyz centroids of 32584 strong reflections from the actual data collection after integration.
1 See Table 1 for additional crystal and refinement information.

Structure solution and refinement
The structure was solved using SHELXT 2 and refined using SHELXL. 3 The space group P-1 was determined based on intensity statistics. Most or all non-hydrogen atoms were assigned from the solution. Full-matrix least squares / difference Fourier cycles were performed which located any remaining non-hydrogen atoms. All non-hydrogen atoms were refined with anisotropic displacement parameters. Ordered hydrogen atoms that participate in hydrogen bonding were found from the difference Fourier map and refined freely. All other hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. The final full matrix least squares refinement converged to R1 = 0.0471 (F 2 , I > 2s(I)) and wR2 = 0.1230 (F 2 , all data).

Structure description
The structure is shown in Figure S2. The asymmetric unit contains one Fe2Zn cluster, four cocrystallized acetone solvent molecules, and one cocrystallized water solvent molecule, all in general positions. Each metal site was modeled as a disorder of Fe and Zn: Fe1:Zn1, 0.89:0.11, Fe2:Zn2, 0.77:0.23, Fe3:Zn3, 0.34:0.66. The total metal ratio was constrained to 2Fe:1Zn. CF3 group C2-F1-F2-F3 is modeled as disordered over two positions (0.58:0.42). One cocrystallized water and two acetone solvent molecules were modeled as disordered over two positions (0.53:0.47); the common disorder ratio is due to their adjacency. Structure manipulation and figure generation were performed using Olex2. 4 Unless noted otherwise all structural diagrams containing anisotropic displacement ellipsoids are drawn at the 50 % probability level. Data collection, structure solution, and structure refinement were conducted at the X-ray Crystallographic Facility, B04 Hutchison Hall, Department of Chemistry, University of Rochester.
The crystal structure of 1 contains co-crystallized solvent molecules, namely acetone and water, that were absent from the crystal characterized in our previous report. Despite the inclusion of these solvent molecules in the structure, the geometry of the Fe2Zn cluster obtained here is very similar to that obtained in our previous work as evidenced by the Fe-Zn representative edge distances within the triangular scaffold of cluster 1 listed in Table S2. We note that prior to using the precursor in a nanocrystal reaction, we dry the crystals at 45 ºC under vacuum. The FTIR spectrum of the dried crystalline product contains peaks associated with coordinated trifluoroacetate ligands, water and the ZnFe2O core but does not contain any peaks associated with acetone ( Figure S3).   Table S2. Comparison of representative edge distances within the triangular scaffold of cluster 1 and a previously reported structure with the same composition but different crystallization solvent molecules.

Lattice Parameter Calculation
The lattice parameters (a) of ZnFe2O4 nanocrystals were determined from the positions of diffraction peaks in the original powder XRD spectrum collected with the Molybdenum X-ray source, using equations S3 and S4.
In these equations, dhkl is the d-spacing, l is the wavelength of the X-ray source used to collect the data, q is the Bragg angle, and hkl are the miller indices associated diffraction peak. We calculated values of lattice parameters from the seven most intense peaks in the XRD spectrum and report their average in Table 2 of the main test. Table S8 contains the complete set of data used in these calculations.  Figure S15. Plot of lattice constant determined from powder X-ray diffraction data versus the inversion parameter x for ZnFe2O4 nanocrystals synthesized in aromatic OH-free solvents.
S a n c h e z -L i e v a n o s e t a l . 19 | 24

Determination of Solubility of 1 in Various Reaction Solvents
The molar absorption coefficient of 1 was determined using four stock solutions at 25ºC diluted to absorbances between 0.1 and 1.0 in accordance with the Beer-Lambert's law. Saturated solutions of 1 were prepared by adding the solid into different solvents at 25ºC, namely benzyl ether, toluene, xylenes and mesitylene, with stirring overnight, and then allowed to settle for at least 1 h. The solutions were then filtered through glass wool to remove any undissolved material. An aliquot of each solution was diluted in its respective solvent and the UV-Vis spectra collected to determine the concentration of 1 in the saturated solution. Given miscibility issues between the solvents of interest and acetonitrile, we decided to record UV-Vis data of the resulting solutions in the solvent they were incubated for 24 hours. Therefore, it is important to note that we assumed the molar extinction coefficient of 1 @340 nm will be basically the same from solvent to solvent in order to perform the solubility analysis. Figure S16. Absorption spectra (left) and associated Beer-Lambert's law plot of absorbance at 340 nm versus concentration of 1 in acetonitrile (right). The dotted line is a linear fit whose slope corresponds to the product of the extinction coefficient, e, and the cell pathlength, l. Since l = 1 cm, this slope indicates that e = 2166 M -1 cm -1 .

Characterization of ligand exchange with cluster 1 by 19 F NMR
We collected 19 F NMR spectra to characterize the starting ZnFe2O cluster (which contains trifluoroacetate ligands), the ligand exchange reaction between the cluster and oleic acid, and the supernatant acquired after purification of the nanoparticles. These spectra are shown in Figure S18. Compared to the 19 F NMR spectraof free trifluoroacetic acid, which contains one narrow resonance at d ~ -76 ppm, the ZnFe2O cluster contains two broader resonances shifted to more positive chemical shifts (d ~ -35 ppm and d ~ -53 ppm). The observed broadening and shifting is consistent with bonding of the trifluoroacetate to a paramagnetic metal center (i.e., Fe 3+ ) and the observation of two peaks is consistent with the two bonding environments occupied by trifluoroacetate ligands in the cluster precursor. Four of the six trifluoroacetate ligands bridge the Zn 2+ center and one of the Fe 3+ centers. We assign the narrower peak at d ~ -53 ppm to these ligands. The other two trifluoroacetate ligands bridge the two Fe 3+ centers. Bonding to two paramagnetic metal centers should increase the peak shift and peak broadening, thus we assign the broader peak at d ~ -35 ppm to these two ligands. Importantly, the spectrum of the cluster does not contain any peaks associated with free trifluoroacetic acid.
After 24 hours in the presence of oleic acid at room temperature (1:OA = 1:108), a narrow peak at a chemical shift similar to that of free trifluoroacetic acid appears in the spectrum alongside the peaks for the bound trifluoroacetate ligands, indicating that the addition of oleic acid displaces trifluoroacetic acid from the cluster. Addition of both oleic acid and oleylamine to the cluster (1:OA:OAm = 1:108:108) produces a 19 F NMR spectrum that contains only free trifluoroacetic acid, indicating that all of the bound trifluoroacetic acid had been displaced from the cluster. Finally, the 19 F NMR spectrum of the supernatant collected after purification of the nanocrystals contains a sharp doublet peak centered at a chemical shift that is slightly more negative than the free trifluoroacetic acid. We hypothesize that this feature may arise from products of the reaction of displaced trifluoroacetic acid with oleylamine (i.e. an amide) and/or with oleic acid (i.e. an acid anhydride). Figure S18. (A) 19 F-NMR spectra of free trifluoroacetic acid (black), the cluster precursor 1 (green), the cluster precursor after 24 hours at room temperature in the presence of oleic acid (dark red) and oleic acid and oleylamine (blue), and of the supernatant collected after purification of the nanocrystals (orange). (B) The same 19 F NMR spectra as shown in A but zoomed in to show the peaks in the region of free trifluoroacetic acid. The spectra of solutions containing cluster 1 were prepared in benzyl ether at the same concentrations used in the nanocrystal reactions. The spectrum of free trifluoroacetic acid was collected in benzyl ether.

Note on the impact 2+ vs 3+ metal charge has on ligand exchange/hydrolysis
There are two competing factors we consider when assessing the relative probability of hydrolysis occurring at M 2+ versus M 3+ sites: the ability of water to access the metal center via displacement of a carboxylate ligand (a kinetic factor) and the relative stability of the product metal hydroxide complex (a thermodynamic factor). The smaller positive charge on the M 2+ center makes it less Lewis acidic than the M 3+ center and results in a weaker bond to the bridging carboxylate ligands. We thus anticipate that the M 2+ center will be more kinetically accessibly to incoming water molecules. However, we also expect the M 2+ ion to form a weaker bond with a hydroxide ligand for similar reasons, which makes it more susceptible to the reverse reaction. Overall, we suspect that these two factors effectively cancel out, resulting in no particular preference for any metal center to be hydroxylated over any other at any given time during the reaction. Figure S21. Characterization of the nanocrystals obtained from reaction of a 1:2 mixture of zinc(II) nitrate and iron(III) nitrate in the presence of oleic acid and oleylamine. (A) A representative TEM micrograph indicates the formation of a polydisperse sample of isotropic nanoparticles. (B) Fe 2p3/2 XPS spectra is consistent the presence of both Fe 3+ (orange peak centered at 711.3 eV) and Fe 2+ (green peak centered at 708.8 eV). (C) The powder x-ray diffraction spectrum of the resulting nanocrystals shows only peaks corresponding to spinel crystal structures. Reference spectra for ZnFe2O4 (JCPDS 01-089-1009) and Fe3O4 (JCPDS 19-1346) are shown to demonstrate that these structures have nearly identical powder X-ray diffraction patterns.